Maintenance & coolant leaks

Alexander, your GT shares the same fragile first-gen DOHC 2.5i that Subaru used in the 1996-1999 era. These motors' heads are over-drilled and have all suffered catastrophic head gasket breaches. So the odds are overwhelming that you've blown the right side HG. Except: the symptoms of such are almost always overheating, first at speed, and eventually during all running. A simple bleeding head indicates more probably an atmospheric (rather than high pressure percolation) HG leak. Nonetheless, there is NO repair short of replacing both HGs to solve the problem. But DO NOTE that if the block has ever been overheated...either from a Mt Vesuvius-like event or chronic percolation...a pricey head gasket replacement ($1.5k) will NOT be successful longterm, as eventually the great majority of these early DOHC 2.5i then blew rods on the highway. So my suggestion is to back away from spending money on this old fragile motor. You CAN swap in a non-GT old 2.2i, but you'll be annoyed at its lack of power, oil leaks, and having to swap out the SCM and maybe exhaust and harness crap too. She's 19 years old: say goodbye.